For three prong electrical plugs used in the United States of America, which are classified by the National Electrical Manufacturing Association as 5-15, two flat prongs are used to conduct an electrical current and a third prong is used as a grounding prong. In the United States of America, there are at least two different kinds of grounding prongs. One type of grounding prong has a U-shaped cross-section. FIG. 1 illustrates a cross-sectional perspective view of a U-shaped grounding prong 36. The width, height, horizontal distance, and vertical distance of the U-shaped prong 36 are defined in this application as illustrated in FIG. 1. A line 19 is a line representing the central axis of the U-shaped grounding prong. The width of the U-shaped grounding prong is the horizontal distance, which is perpendicular to the central axis of the U-shaped grounding prong, from the outside of a first foot 21 of the U-shaped grounding prong to the outside of a second foot 23 of the U-shaped grounding prong. The U-shaped cross-section has a width 22 of 0.184 inches and a height 20 of 0.200 inches. The height of the U-shaped prong is defined as the vertical distance from the bottom of the first foot 21 of the U-shaped grounding prong to the top of the arc 25 of the U-shaped grounding prong. The vertical distance is the distance perpendicular to the horizontal distance and the central axis of the grounding prong. In the U-shaped grounding prong shown in FIG. 1 the top of the arc 25 is equidistant from the bottom of the first foot 21 and the bottom of the second foot 23 of the U-shaped grounding prong 36. A second type of grounding prong is a tubular grounding prong which has a circular cross-section. A cross-sectional perspective view of the tubular grounding prong 30 is illustrated in FIG. 2. In the United States the diameter 24 of the circular cross-section of the tubular grounding prong is 0.184 inches.
FIG. 3 is an illustration of a cross-sectional perspective view of one embodiment of a prior art grounding prong socket 28 with a tubular grounding prong 30 placed inside. The socket has a cross-section similar to the cross-section of the U-shaped prong, but is made for the dimensions of the tubular prong. A seam 26 is placed at the top of the socket. FIG. 4 is an illustration of the cross-sectional perspective view of the prior art embodiment 28 shown in FIG. 3 with a U-shaped prong 36 inside the prior art socket 28. Seam 34 creates a much wider gap, indicating that the prior art socket 28 is stretched to accommodate the U-shaped prong with larger dimensions than the tubular prong. Because the prior art socket 28 is stretched, when the U-shaped prong is removed and the tubular prong is inserted, the stretched socket no longer firmly fits the tubular prong, causing improper grounding, which could cause an electrical shock.
In the prior art there are other embodiments of grounding sockets. These sockets stretch or bend for the larger U shaped prong, causing improper contact between the stretched socket and the smaller tubular prong. Other examples of prior art grounding prong sockets are disclosed in U.S. Pat. Nos. 3,467,939, 3,891,289, and 4,018,492 which are herein incorporated by reference.